The present invention relates to a clamp or ultrasonic transducer and to a novel ultrasonic coupling compound which is physically and chemically stable over a large temperature range for coupling the transducer crystal to the transducer housing, and for coupling the housing to a conduit.
A coupling medium, such as a grease or the like, is commonly used to insure that ultrasonic energy can be transmitted between a crystal and a transducer housing and the structure to which the housing is connected. The coupling medium should not degrade rapidly in its sonic conduction ability and should remain in place and stable in the space between the surface being coupled over a wide temperature range, for example, from very low, cyrogenic temperatures to about 500.degree. F., or to the temperature limit of the transducers. Ultrasonic transducers are frequently clamped to surfaces in an environment in which the coupling material will be subject to high temperature, vibration and other harsh environmental conditions. For example, "clamp-on" ultrasonic flowmeters which monitor fluid flow as disclosed in either of Baumoel U.S. Pat. No.s 3,987,674 or 4,373,401 or fluid level as shown in Baumoel U.S. Pat. No. 4,144,517 may be subjected to temperatures from cryogenic to 500.degree. F. or higher due to extremely hot fluids within the conduit. Presently available ultrasonic coupling greases are unsuitable for such conditions and have a relatively short life. For example, the commonly available colloidal grease type couplants will exhibit an excessive degree of thermal outgassing with eventual loss of physical properties and loss of sonic coupling. Such changes cause loss of ultrasonic coupling which could result in a failure of the equipment. Due to outgassing, chemical deterioration, or changed molecular crosslinking, the in-service life of common couplants is very unsatisfactory at high temperatures. Because of this relatively short predicted service life, frequent changing of the couplant is necessary. Even where the couplant is used at lower temperature and exhibits slower physical and chemical change, it must be changed occasionally, resulting in down time and maintenance expenses.
A typical prior art couplant material is DOW-CORNING 340 Heat Sink Compound. This material is believed to be described in U.S. Pat. No. 4,738,737 and is a grease-like silicone fluid heavily filled with zinc oxide, used as an acoustic couplant material under high-temperature and high-radiation conditions. However, it has been found that such a material does not maintain its sonic properties for a long enough time to avoid numerous changes of couplant.
Accordingly, there has been a long-standing need for a couplant which:
1. provides required acoustical properties in the form of a sound path with good acoustical impedance between a crystal, its housing and the pipe or other sonic medium to which the housing is attached;
2. withstands thermal cycling from room temperatures, and in some cases, to temperatures over 500.degree. F.;
3. maintains stable acoustical properties for periods of time which may be as great as several years between planned maintenance intervals;
4. emits a minimum of irritating fumes and does not outgas and disturb the sonic path;
5. has a viscosity which provides for ease of application and use; and
6. does not require expensive surface preparation of the conduit or pipe surfaces in the field, such as grinding or machining.
In search of a couplant which would answer all of these long-standing needs, various materials which were available commercially for other uses were studied. The intent was to identify a material that did not have objectionably high contents of sulfur or lead or other toxic or noxious material; had the ability to perform the ultrasonic couplant function at temperatures commonly encountered in flow meter applications; had a viscosity which was suitable for easy use as an ultrasonic couplant; and did not have hazardous or irritating fumes.
Fluoroether greases were identified as satisfactory couplant materials, particularly two specific fluoroether greases; Nye Fluoroether Grease 849 and Dupont Krytox.RTM. grease. These greases are known fluoroether lubricants; however, nothing suggests that fluorinated ethers, and particularly the Nye Fluoroether Grease 849 and Dupont Krytox.RTM. grease lubricants would be stable acoustical coupling greases operable over a wide temperature range for a long time without physical or chemical changes.